KR20100076727A

KR20100076727A - High strength steel sheet for pressure vessel with excellent hic and fatigue resist properties and manufacturing method thereof

Info

Publication number: KR20100076727A
Application number: KR1020080134863A
Authority: KR
Inventors: 홍순택
Original assignee: 주식회사 포스코
Priority date: 2008-12-26
Filing date: 2008-12-26
Publication date: 2010-07-06

Abstract

PURPOSE: A high strength steel sheet for a pressure vessel with excellent HIC and fatigue resist properties and a manufacturing method thereof are provided to fatigue property and improve tensile strength by adding Ti and N. CONSTITUTION: A high strength steel sheet for a pressure vessel comprises C 0.10~0.30 weight%, Si 0.15~0.40 weight%, Mn 0.6~1.2 weight%, Al 0.001~0.05 weight%, P 0.035 weight% or less, S 0.020 weight% or less, Cr 0.35 weight% or less, Mo 0.2 weight% or less, Ni 0.5 weight% or less, V 0.05 weight% or less, Nb 0.05 weight% or less, Ca 5~50ppm, Ti 0.005~0.025 weight%, N 0.0020~0.0060 weight%, and the rest including Fe and inevitable impurities. Cu+Ni+Cr+Mo are included less than 1.5 weight%. Cr+Mo are included less than 0.4 weight%. V+Nb are included less than 0.1 weight%.

Description

High Strength Steel Sheet for Pressure Vessel with Excellent HIC and Fatigue Resist Properties and Manufacturing Method Thereof}

The present invention relates to a pressure vessel steel plate having excellent HIC characteristics and fatigue strength, and to a method of manufacturing the same, and more particularly, to fine Nb (Ti) (C, N) having an average size of 50 nm or less by adding a small amount of Ti and N. Steel sheet having excellent hydrogen organic cracking resistance (HIC characteristics) and fatigue characteristics, which can improve the fatigue strength of the pressure vessel steel sheet by precisely controlling the volume fraction (%) to 0.3-0.5% It relates to a manufacturing method.

As the recent shortage of oil and high oil prices have resulted in the development of low quality crude oil, which has not been used before, and the development of poor oil fields under active conditions, the hydrogen of refined and hydrogenated steels containing wet hydrogen sulfide has been actively developed. It is required to increase resistance to hydrogen induced cracks (HIC).

The generation principle of hydrogen induced crack (HIC) is as follows. When the steel sheet causes a corrosion reaction with a wet hydrogen sulfide atmosphere, hydrogen is generated, and the generated hydrogen penetrates into the steel in an atomic state and then diffuses to form molecules in inclusions in the steel. When hydrogen is molecularly formed in an atom, it becomes a form of hydrogen gas, and gas pressure is generated, and the pressure causes cracking and growth of tissue.

In order to improve the resistance to such hydrogen organic cracking (hereinafter referred to as 'HIC resistance' in the present specification), in the prior art, ① a method of adding Cu and / or Co, ② a method of reducing impurities and inclusions, and controlling the shape, (3) A method of suppressing hydrogen intrusion or diffusion by fine dispersion of carbonitride has been proposed.

However, since the prior art ① has to add an expensive alloy element, such as Co, there is a problem that the manufacturing cost can be increased rapidly, and the prior art ② may cause a load of steelmaking operation. In addition, the prior art (3) is composed of a structure in which the microstructure of the steel is ferrite and pearlite, the control of such a structure is practically limited, there is a problem that it is difficult to apply to a practical process.

The present invention is to solve the problems of the prior art and to produce steel for pressure vessels with excellent HIC and fatigue characteristics, according to the Banding Index (ASTM E-1268) of the microstructure composed of ferrite and perlite by recrystallization controlled rolling. Measurement), and the addition of Ti and N to provide a tensile strength 500MPa class steel sheet for pressure vessel and its manufacturing method.

In the present invention, by weight%, C: 0.10 to 0.30%, Si: 0.15 to 0.40%, Mn: 0.6 to 1.2%, Al: 0.001 to 0.05%, P: 0.035% or less, S: 0.020% or less, Cr: 0.35% or less, Mo: 0.2% or less, Ni: 0.5% or less, V: 0.05% or less, Nb: 0.05% or less, Ca: 5-50 ppm, Ti: 0.005-0.025%, N: 0.0020-0.0060%, balance Fe And other unavoidable impurities

Cu + Ni + Cr + Mo: 1.5% or less;

Cr + Mo: 0.4% or less;

V + Nb: 0.1% or less; And

Ca / S: 1.0 or less

Satisfy the relationship of,

It provides a high-strength pressure vessel steel sheet, characterized in that the volume fraction (%) of Nb (Ti) (C, N) precipitates of 50 nm or less is 0.3 to 0.5%. In this case, the steel sheet may have a banding index of 0.25 or less and a volume fraction (%) of Nb (Ti) (C, N) precipitates of 50 nm or less is adjusted to 0.3 to 0.5% to obtain excellent fatigue characteristics and HIC resistance. Can be. In addition, the microstructure of the steel sheet may be a ferrite + pearlite two-phase composite structure.

Further, the present invention, the slab of the above-described component is heated to 1050 ~ 1250 ℃, rolling at a temperature of Tnr ~ Tnr + 100 ℃, normalizing 1.3 * t + (10-30 minutes) at 850 ~ 950 ℃ and 600 PWHT treatment at ~ 640 ℃ provides a high strength pressure vessel steel sheet manufacturing method excellent in HIC characteristics and fatigue properties.

According to the present invention, it is possible to easily provide a steel plate for a high-strength pressure vessel equipped with excellent HIC characteristics and fatigue characteristics at the same time and a manufacturing method thereof with economical and high productivity.

The present invention controls the banding index (measured according to ASTM E-1268) of the microstructure composed of ferrite and perlite to a range of 0.25 or less, and adds a small amount of Ti and N to improve the fatigue strength. Main content is to control the volume fraction (%) of fine Nb (Ti) (C, N) to 0.3-0.5%.

Hereinafter, the component system which comprises this invention is demonstrated in detail.

C: 0.10 to 0.30%

In the present invention, C is limited to 0.10 to 0.30% as an element for securing strength. If the content of C is less than 0.10%, there is a problem in that the strength of the matrix on the matrix may be lowered, thereby lowering the desirable property.

Si: 0.15 ~ 0.40%

Si is an alloying element added for the deoxidation effect, the solid solution strengthening effect, and the impact transition temperature raising effect, and at least 0.15% is added. However, if the content exceeds 0.40%, the weldability is lowered and the oxide film may be severely formed on the surface of the steel sheet, so Si is added at 0.15 to 0.40%.

Mn: 0.6 ~ 1.2%

Excessive addition of Mn forms MnS, which is a non-metallic inclusion drawn together with S, thereby lowering room temperature elongation and low temperature toughness, thereby managing Mn to 1.2% or less. However, when Mn is less than 0.6% due to the component properties of the present invention, it is difficult to secure appropriate strength, so the amount of Mn added is limited to 0.6 to 1.2%.

Al: 0.001-0.05%

Al, together with Si, is one of the strong deoxidizers in the steelmaking process to add 0.001% or more to achieve this effect. However, if the content is added in excess of 0.05%, the effect is saturated and the manufacturing cost is increased, so Al is limited to 0.001 to 0.05%.

P: 0.035% or less

Since P is an element that impairs low temperature toughness, it is better to manage it as low as possible, but to remove it excessively in the steelmaking process is expensive, so it is managed within 0.035% or less.

S: 0.020% or less

S is also an element that adversely affects low temperature toughness along with P, but like P, it may be excessively expensive to remove in the steelmaking process, so it is appropriate to manage it within 0.020% or less.

Cr: 0.35% or less (excluding 0%)

Cr is an alloying element that can increase the strength, but is an expensive element, and if it is added in excess of 0.35%, Cr causes an increase in manufacturing cost, so it is limited to within 0.35%.

Mo: 0.2% or less (except 0%)

Mo is an alloying element that is effective for improving strength, such as Cr, and is known as an element that prevents cracking caused by sulfides. However, Mo is also an expensive element, it is preferable to add in the range of 0.2% or less from the economical point of view.

Ni: 0.5% or less (except 0%)

Ni is added to the present invention as an element effective in improving low temperature toughness, but Ni is also added as an expensive element at an economical level of 0.5% or less.

V: 0.05% or less (excluding 0%)

V is an element effective for increasing the strength, such as Cr and Mo, but is preferably added within 0.05% because of its high price.

Nb: 0.05% or less (excluding 0%)

Nb is dissolved in austenite to increase the hardenability of austenite, and also precipitates as carbonitride ((Nb, Ti) (C, N)) that matches the matrix with Ti to increase the strength of the steel. Acts as an important element However, when Nb is added in an excessively large amount, it may appear as a coarse precipitate in the playing step and act as a site of hydrogen organic cracking, so the content of Nb in the present invention is limited to 0.05% or less.

Ca: 5 ~ 50ppm

Ca is produced by CaS serves to suppress the non-metallic inclusions of MnS, for this purpose is added 5ppm or more. However, if the amount is excessive, the upper limit is set to 50 ppm because it reacts with O contained in the steel to form CaO, which is a non-metallic inclusion, which is not good for physical properties.

Ti: 0.005-0.025%

The appropriate amount of Ti may vary somewhat depending on the content of Nb and N. If the amount of Ti added is relatively small compared to the amount of N, the amount of (Nb, Ti) N is reduced, which is detrimental to the refinement of grains, whereas when the amount is added in excess, (Nb, Ti) N becomes coarse during the heating process. Thus, the effect of inhibiting grain growth is rather reduced. Therefore, the amount of Ti is generally limited to 0.005 to 0.025% in consideration of the N content (20 to 60ppm) contained.

N: 0.0020 to 0.0060% (20 to 60 ppm)

N forms a (Nb, Ti) (C, N) precipitate together with Nb and Ti to refine the grains of the steel, thereby increasing the toughness of the base metal and the impact toughness of the HAZ portion. To this end, in the present invention, the amount of N added is limited to 0.0020 to 0.0060% in consideration of the content of Nb and Ti. The addition of N in excess of 0.0060% may cause an excessive increase in the amount of (Nb, Ti) (C, N) produced and lower the low temperature toughness.

Cu + Ni + Cr + Mo: 1.5% or less

Cr + Mo: 0.4% or less

V + Nb: 0.1% or less

Ca / S: 1.0 or less

The relationship between Cu + Ni + Cr + Mo, Cr + Mo, and V + Nb is a value limited by the basic standard for steel for pressure vessels (ASTM A20), and accordingly, the Cu + Ni + Cr + Mo content is 1.5%. Hereinafter, the Cr + Mo content is limited to 0.4% or less, and the V + Nb content is limited to 0.1% or less.

And the ratio of Ca / S is an essential constituent ratio to improve the hydrogen organic crack resistance by spheroidizing the MnS inclusions, and since the effect is difficult to be expected at 1.0 or less, the ratio is adjusted to exceed 1.0.

Hereinafter, the microstructure constituting the present invention will be described in more detail.

Microstructure: Ferrite + Perlite two-phase composite structure, Banding Index controls the volume fraction (%) of Nb (Ti) (C, N) of 0.25 or less and 50nm or less to 0.3 ~ 0.5%

In the present invention, a steel material including the above-described alloying elements and satisfying the relationship is subjected to normalizing to form a ferrite + pearlite two-phase composite structure. In this case, in the present invention, the composite tissue should be treated to form a microstructure having a banding index (measured according to ASTM E-1268) of 0.25 or less. If the banding index value exceeds 0.25, the HIC resistance of the microstructure may be sharply deteriorated, so be careful.

Hereinafter, the method of manufacturing the steel sheet of the present invention will be described in more detail.

In order to efficiently control the microstructure of the inventive steel composed of the above composition, it is necessary to perform a heat treatment to secure an appropriate tensile strength of 500 MPa after the hot rolling method (recrystallized controlled rolling) and PWHT (Post Weld Heat Treatment).

Reheating Temperature: 1050 ~ 1250 ℃

In the present invention, the slab having the composition described above is reheated at 1050 to 1250 ° C. If the reheating temperature is lower than 1050 ℃, it is difficult to solute the solute atoms, while if the reheating temperature exceeds 1250 ℃ austenite grain size becomes too coarse to reduce the properties of the steel sheet.

Recrystallization controlled rolling: 50% or more cumulative rolling reduction at a temperature of Tnr to Tnr + 100 ° C and a rolling reduction of 10% or more per each rolling pass

The recrystallized controlled rolling means hot rolling at a temperature higher than the _{uncrystallized crystal} , and the recrystallized temperature, T _nr , can be calculated through Equation 1 below. However, in the formula, the unit of each alloy element represents weight%. The rolling reduction rate and the cumulative rolling reduction per rolling pass are carried out under such rolling conditions for the structure refinement to the conditions necessary for securing a banding index value to be described later.

[Equation 1]

Tnr (° C) = 887-464 × C + 890 × Ti + 363 × Al-357 × Si + (6445 × Nb-644 × Nb ^1/2 ) + (732 × V-230 × V ^1/2 )

Banding Index: below 0.25

Recrystallization controlled rolling is the most important variable in order for the banding index value to be 0.25 or less, and the recrystallization controlled rolling has a cumulative reduction of 50% by applying a reduction ratio of 10% or more for each rolling pass in the temperature range of T _nr to T _nr + 100 ° C. It is essential to give more than%. If the microstructure having a banding index value of 0.25 or less is formed, the HIC characteristics required by the present invention can be provided.

The hot-rolled and cooled steel sheet can secure the physical properties required by the present invention only when the tensile strength of 500 MPa and -50 ° C. is at least 50 Joules. To this end, normalizing heat treatment and PWHT are performed under the following conditions. .

Normalizing condition: 1.3 * t + (10-30 minutes) at 850 ~ 950 ℃

Normalizing heat treatment is performed at 850-950 degreeC on condition of 1.3 * t + (10-30 minutes) (t means thickness of steel (mm)). If the normalizing temperature is lower than 850 ° C, it is difficult to re-use the solid solute elements, and thus, it is difficult to secure the strength. On the other hand, if the normalizing temperature is higher than 950 ° C, grains grow to damage low-temperature toughness. In addition, the reason for the limitation of the heat treatment time is that the homogenization of the tissue is difficult when less than the reference time, and the productivity is impaired when the time is kept longer.

PWHT conditions: 600-640 ° C for 3 hours per inch thickness

After manufacturing the pressure vessel by welding the steel which has been normalized, PWHT treatment is required. PWHT temperature conditions are carried out at 600 ~ 640 ℃. This is because when the PWHT temperature is lower than 600 ° C., residual stress such as welding is not smoothly removed. When the PWHT temperature is higher than 640 ° C., the strength of the steel may decrease. The time condition of PWHT is preferably for 3 hours per inch thickness, because less than the reference time makes it difficult to remove residual stress.

The volume fraction of fine precipitates Nb (Ti) (C, N) of 50 nm or less precipitated in the matrix through a series of heat treatment exaggerations such as recrystallized controlled rolling, normalized heat treatment, and PWHT for slabs having the component system described above is 0.3 to 0.5. It is possible to produce a tensile strength 500MPa grade steel sheet having excellent HIC resistance and fatigue properties, which is%.

Hereinafter, the present invention will be described in more detail with reference to Examples.

(Example)

Table 1 shows the chemical components of the inventive steel and the comparative steel, respectively. The steel slab of the inventive material having the alloy composition as shown in Table 1 was heated at an appropriate temperature range and 55 to 80% of the recrystallization controlled rolling was carried out in the recrystallization zone to control the banding index to 0.25 or less.

Steel grade C Mn Si P S Al Ni Cr Mo V Nb Ti N Ca a 0.17 1.11 0.28 0.010 0.0015 0.02 0.18 0.08 0.11 0.008 0.015 0.015 0.0030 0.0020 b 0.17 1.02 0.27 0.080 0.0012 0.01 0.15 0.09 0.10 0.011 0.014 0.010 0.0025 0.0025 c 0.17 1.05 0.28 0.010 0.0017 0.03 0.18 0.15 0.08 0.010 0.010 - 0.0032 0.0025 d 0.14 1.15 0.29 0.012 0.0014 0.02 0.15 0.20 0.15 0.009 0.012 - 0.0034 0.0023

In addition, normalization and PWHT were performed under the conditions shown in Table 2, and then yield strength, low temperature toughness, and crack length ratio (%) were examined. However, low-temperature toughness was evaluated by the Charpy impact energy value obtained by performing a Charpy impact test on a specimen having a V notch at -50 ° C, and the crack length ratio (%) was measured according to the NACE TM0277 standard.

Furthermore, the volume fraction (%) of precipitate Nb (Ti) (C, N) was measured by electron microscopy and the extraction of precipitate residues. It was. The experimental results are summarized in Table 3 below.

division Grater
thickness
(mm) Slab
heating
Temperature
(℃) Recrystallization
Control rolling
Cumulative rolling reduction
(%) Normalizing
Temperature
(℃) Normalizing
time
(minute) PWHT
Temperature
(℃) PWHT
time
(hr) invent a 13 1200 60 900 50 620 3 25 1180 75 890 60 620 3 50 1120 55 905 80 610 6 50 1120 70 910 85 610 6 b 70 1100 80 910 110 610 9 75 1100 85 910 120 610 9 80 1100 60 890 120 610 12 Comparative material c
20 1200 20 900 60 620 3 25 1150 30 900 50 620 3 d 50 1100 40 900 80 620 6

division Banding
Index YS
(Mpa) TS
(Mpa) -50 ℃
Impact toughness
(J) Of Nb (Ti) (C, N)
Volume fraction (%) * fatigue
burglar** CLR
(%) foot
persons
ashes a 0.14 378 542 206 0.45 0.59 0.0 0.13 370 543 200 0.48 0.58 0.0 0.16 365 537 203 0.42 0.60 0.0 0.14 357 536 198 0.43 0.61 0.0 b 0.11 358 539 201 0.40 0.59 0.0 0.12 356 534 198 0.41 0.58 0.0 0.14 353 529 179 0.39 0.60 0.0 Comparative material c
0.37 370 539 186 - 0.50 55 0.30 365 530 175 - 0.49 60 d 0.35 358 520 190 - 0.48 35

* Volume fraction (%) of fine Nb (Ti) (C, N) having a size of 50 nm or less: Volume fraction (%) of fine precipitates Nb (Ti) (C, N) having a size of 50 nm or less

** Fatigue Strength: Fatigue ratio value at Fatigue Limit in S-N curve of high cycle fatigue test

As shown in Table 3, the yield strength, tensile strength, and low temperature toughness of the invention and the comparative material showed almost the same level, but the CLR (Crack Length Ratio,%) and the resistance under H ₂ S (sour gas) gas atmosphere and Fatigue strength can be seen that the invention material is excellent.

As such, the reason why the invention is excellent in the CLR is because the banded index indicating the homogenization degree of the microstructure composed of ferrite + pearlite is controlled to be 0.25 or less and the volume of fine Nb (Ti) (C, N) of 50 nm or less. It can be seen from this example that the fraction was achieved by controlling 0.3-0.5%.

Claims

By weight%, C: 0.10 to 0.30%, Si: 0.15 to 0.40%, Mn: 0.6 to 1.2%, Al: 0.001 to 0.05%, P: 0.035% or less, S: 0.020% or less, Cr: 0.35% or less ( 0% excluding), Mo: 0.2% or less (excluding 0%), Ni: 0.5% or less (excluding 0%), V: 0.05% or less (excluding 0%), Nb: 0.05% or less (0% Ca: 5-50 ppm, Ti: 0.005-0.025%, N: 0.0020-0.0060% and the balance Fe and other unavoidable impurities

Cu + Ni + Cr + Mo: 1.5% or less;

Cr + Mo: 0.4% or less;

V + Nb: 0.1% or less; And

Ca / S: 1.0 or less

Satisfy the relationship of,

A steel sheet for high-strength pressure vessels, characterized in that the volume fraction (%) of Nb (Ti) (C, N) precipitates of 50 nm or less is 0.3 to 0.5%.

The steel sheet for high-strength pressure vessel of claim 1, wherein the steel sheet has a banding index of 0.25 or less.

The steel sheet for high-strength pressure vessel according to claim 1 or 2, wherein the microstructure of the steel sheet is a ferrite + perlite two-phase composite structure.

According to claim 1 or claim 2, wherein the steel sheet is a high strength pressure vessel steel plate, characterized in that the tensile strength is 500MPa or more and the impact toughness of -50 ℃ more than 50 Joules.

Cu + Ni + Cr + Mo: 1.5% or less;

Cr + Mo: 0.4% or less;

V + Nb: 0.1% or less; And

Ca / S: 1.0 or less

For steel slabs that satisfy the relationship of

Reheating step to heat to 1050 ~ 1250 ℃;

A recrystallization zone controlled rolling step of rolling at a temperature of Tnr ˜Tnr + 100 ° C .;

A normalizing step of normalizing at 1.3 * t + (10-30 minutes) at 850-950 ° C (where t denotes the thickness of the steel sheet); And

PWHT step of PWHT treatment at 600 ~ 640 ℃

Method for producing a steel plate for high-strength pressure vessel comprising a.

(Tnr (° C) = 887-464 × C + 890 × Ti + 363 × Al-357 × Si + (6445 × Nb-644 × Nb ^1/2 ) + (732 × V-230ⅹV ^1/2 ))

The method according to claim 5, wherein the recrystallization zone controlled rolling step comprises a rolling reduction ratio of 10% or more and a cumulative reduction of 50% or more per rolling pass.

7. The method of claim 5 or 6, wherein the PWHT is performed for 3 hours per 1 inch thickness.